1. Field of the Invention
The present invention is concerned with velocity head recovery fan cylinders for use on water cooling towers in order to provide an increased measure of head recovery in the air currents drawn through and propelled from the tower, so that fan power requirements are lessened. More particularly, it is concerned with such a recovery fan cylinder which is configured such that the section thereof above the fan blade includes respective portions which are cooperatively configured to define therebetween a reflex angle of greater than 180.degree.. Such a construction gives enhanced velocity head recovery and is therefore highly desirable.
2. Description of the Prior Art
Relatively large industrial sized induced draft water cooling towers of either the crossflow or counterflow variety have long been provided with velocity head recovery stacks which are mounted in circumscribing relationship to the powered fan(s) associated with the towers and extend upwardly therefrom. The purpose of such stacks is twofold. First, such stacks serve to discharge and guide hot exhaust air to a position above the tower where it diffuses into the ambient atmosphere and is carried away from the cool air inlet of the tower by the prevailing winds. It is necessary to discharge hot discharge air at an elevation where recirculation of such air back through the cool air inlets of the tower is prevented, since recirculation measurably lowers cooling efficiency. Second, stacks lessen fan horsepower requirements by virtue of "recovery" of pressure of air discharged therethrough, such occurring because of the diverging contour of the stacks.
Recovery stacks are generally configured with a venturi-like restriction intermediate the ends thereof which surrounds as closely as possible the fan blade, along with a divergent upper discharge section above the fan blade in which reduction in air velocity and partial recovery of pressure occurs. As noted, such a stack configuration serves to lessen fan power requirements, and in large towers the savings can be significant.
A typical prior art velocity head recovery stack is disclosed in U.S. Pat. No. 3,780,999. As illustrated in this patent, the stack includes an eased inlet section, a venturi-like intermediate fan-receiving section, and a divergent recovery section thereabove. Experience with prior stacks of this type has demonstrated that the most efficient recovery obtains when the tubular sidewall above the fan blade diverges at a substantially constant angle of 7.5.degree. relative to the central axis of the stack. Although from a theoretical standpoint greater angles of divergence should be more effective, in practice it has been found that significantly larger angles of divergence in such conventional stacks results in unacceptable inefficiencies, stemming from frictional losses and the inability of air leaving the fan to adequately fill the relatively large stack volume prior to discharge to the atmosphere.
Those skilled in the art will readily appreciate that any improved stack design leading to enhanced velocity head recovery would be a major breakthrough in the art. This is particularly the case in view of spiralling energy costs, and the fact that an improved stack design could potentially aid in lowering such costs.